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I have some questions regarding reinforcement of garage slabs adjacent to poured concrete basement walls.

I am planning to build a retirement home with a three car attached garage. Two of the garages will be used as a shop, the other for a car. Construction procedures I plan t0 specify:
1. Insulate all garage slabs with 2" thick ESP foam board (R-10) + add 6 mil polyethylene under foam for moisture barrier.
2. Will specify re-bar grid in slab to prevent cracking + install side expansion bars adjacent to each garage slab.
3. Should I also tie the slab into the poured concrete foundation wall using rebar....or should the slab be free to float? ( Frost line in area (Bucks County PA is 3 feet)
4. Should I specify "air entrained" concrete for both the slab and the basement walls? I understand air entrained concrete limits cracking and possibly increase the R value of the 4" concrete slab and/or the 12" basement walls??

Thanks, Bob

Asked by Robert Weber
Posted Dec 5, 2012 10:41 AM ET
Edited Dec 9, 2012 5:04 AM ET


2 Answers

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Putting 2" of extruded polystyrene under the slab has a serious greenhouse gas down-side. It's far better for the environment to use 2.5" of EPS (the white beaded stuff) to hit your R10. Even though 1.5lb density Type-II goods would be adequate for the application, some local codes require 2lb density Type IX EPS, which is of equal or better compressive rating than 1.5lb XPS.

Either way it's not going to be much of a cost-adder- it's usually cheaper, and since it's blown with pentane rather than HFC134a it has about 0.5% of the greenhouse gas potential of XPS at any given R value.

There's no down-side to floating the slab from the foundation walls, and if you put an inch or more of EPS to isolate the slab from the foundation the side stresses on the foundation from thermal expansion are lower (you'd be advised to put some amount of expansion joints in the slab anyway, and EPS is fine.) Building the stem-walls (and basement walls) with insulating concrete forms would get you there automatically.

Bucks county is on the cool edge of US climate zone 4/warm edge of zone 5, and a minimalist R16 ICF approach to the foundation wall insulation would still make longer-term economic sense, along with a couple inches of EPS under the basement slab, reducing mold potential on any good stored in the basement.

Answered by Dana Dorsett
Posted Dec 5, 2012 3:23 PM ET


Hi Dana: Thanks for your response!

Question: Is the down-side of using 2" of "Extruded" Polystyrene foam board under the slab related to the greenhouse gas e.g. HFC134, that is used to manufacture the rigid EPS foam board??

Also, are you recommending the use of "Expanded" Polystyrene Foam Board as an alternate to the "Extruded" version since it's manufacturing process has less of an impact on the environment? (I think the Expanded version has a lower R value per inch than the Extruded version.)

Also, can I assume that if EPS foam board is installed either below the slab or adjacent to to the exterior of the basement walls, there will be no out gassing problems and the R-value will remain fairly constant over the life of the insulation, e.g. 30 years or more?

Thanks also for your suggestion about floating the garage slabs vs. connecting them via rebars to the foundation wall. Sounds like the floating arrangement will allow movement and expansion without stressing the foundation wall.

Any thoughts about using air entrained concrete; somewhere I read it helped eliminate cracks in concrete due to frost heaving and perhaps the "trapped" air in the concrete might also help increase the low R value of concrete somewhat?

Thanks, Bob Weber
PS FYI: I am a retired from the College of New Jersey (2005) and currently in the process of planning my retirement home (Passive Solar with Sunspace) in Bucks County PA. Working with an architect, and other energy & building consultants and expect a finished set of plans that will be ready for builder bids in Jan. 2013. Not sure what your background is but I certainly appreciate your advice....Thanks again!

Answered by Robert Weber
Posted Dec 8, 2012 12:34 PM ET

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